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Historically, the measurement of the extragalactic gamma-ray background (GRB) was beset by great experimental difficulties. The high charged particle intensities in the balloon and space environment produce an intense background in gamma-ray instruments. Most early instruments had large fields of view and poor spatial resolution, which added complexities to the extraction of the diffuse extragalactic component. Data were obtained by many balloon- and satellite-borne instruments the last 30 years (Table 1), with a gradual improvement in the detector sensitivity achieved by larger detector sizes and better background-reduction techniques.

Table 1.
Observations of the Diffuse Gamma-Ray Background

(adapted from Gruber (1992))

Energy Band Detector Platform Reference

0.1-3 MeV scintillator Ranger 3 Arnold et al. (1962)
1-6 MeV scintillator ERS18 Vette et al. (1970)
1-5 MeV scintillator balloon Vedrenne et al. (1971)
0.1-8.5 MeV scintillator balloon Daniel et al. (1972)
0.03-4.1 MeV scintillator Kosmos Mazets et al. (1975)
0.1-4 MeV scintillator balloon Fukuda et al. (1975)
0.3-10 MeV scintillator Apollo Trombka et al. (1977)
2-10 MeV Compton telescope balloon White et al. (1977)
0.3-6 MeV scintillator balloon Mandrou et al. (1979)
3-60 keV proportional counter HEAO-1 A2 Marshall et al. (1980)
1.1-10 MeV scintillator balloon Schönfelder et al. (1980)
0.013-4.0 MeV scintillator HEAO-1 A4 Gruber et al. (1985)
> 50 MeV scintillator/Cerenkov OSO-3 Kraushaar et al. (1972)
30-50 MeV spark chamber balloon Pinkau et al. (1973)
35-200 MeV spark chamber SAS-2 Fichtel et al. (1978)
4-100 MeV spark chamber balloon Lavigne et al. (1982)

Though some of the earlier results gave only upper limits, we have now measured with reasonable accuracy the general form and magnitude of the GRB spectrum (Fig. 1). The low-energy portion (10 keV to 60 keV) of the GRB is characterized by a bremsstrahlung spectral form that can be approximated by a power-law segment of energy index ~ 0.4. The energy spectrum transitions to a power law of index ~ 1.6 above 60 keV, flattens to an index of ~ 0.7 around 1 MeV, then steepens again above several MeV to an index of ~ 1.7 that applies to energies well above 100 MeV. When plotted in intensity per logarithmic energy interval EIE (Fig. 2), the GRB spectrum exhibits two peaks, one at ~ 30 keV and the other at a few MeV (the MeV bump) (Zycki et al. 1993). There is still the lack of good measurements around the 10 to 30 MeV range due to the extraordinary high instrumental and atmospheric background in that regime. Results from the GRB observations by the Compton Gamma Ray Observatory (Compton) now in orbit will eventually fill in this range, and also vastly improve our knowledge at higher energies.

Figure 1

Figure 1. Energy spectrum of the diffuse extragalactic background (Gruber 1992).

The GRB is of particular interest to cosmology because of the transparency of the Universe to gamma rays back to redshifts of 100 or more. A determination of the isotropy of the GRB can be a sensitive test of its cosmological origin and a detailed study of the complete GRB spectrum and its spatial fluctuations may provide constraints to different cosmological models. There are two possibilities for the origin of the extragalactic GRB: (1) it is truly diffuse; and (2) it is the integrated emission of various distant unresolved gamma-ray sources. It may also be a combination of diffuse and point sources and may have different origins in different portions of the gamma-ray band. From below 10 MeV to 100 MeV, particle-antiparticle annihilation, bremsstrahlung and inverse Compton interactions between cosmic ray particles and lower-energy photons are the most likely gamma-ray production mechanisms. Above 100 MeV, the dominant process is pi° decay from nucleon interactions.

Figure 2

Figure 2. The extragalactic background spectrum in intensity per logarithmic energy interval (adapted from Zycki et al. 1993). Circles correspond to the data points compiled by Gruber (1992) in Fig. 1. The elongated 0.5-2 keV error contour gives the ROSAT results (Hasinger 1992). The dashed region is the systematic error contour for the Apollo results, with the middle dot-dashed curve the best estimate of the spectrum. The 35-100 MeV error contour is the SAS-2 result from Fichtel et al. (1978).

In this paper we summarize the state of our knowledge on the extragalactic GRB. We present the observations and their interpretations in two parts: (1) for energies from 0.1 to 10 MeV, and (2) for energies greater than 10 MeV. We then present the recent results from AGN observations by Compton and discuss their relevance to the GRB.

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